The 3rd Generation Partnership Project (3GPP) Release 12 specifies an X2 Gateway (X2 GW) (see, for example, Non-patent Literature 1). As described in Non-patent Literature 1, the X2 GW establishes a signaling (i.e., Stream Control Transmission Protocol (SCTP)) connection with each of a plurality of (H)eNBs and these (H)eNBs exchange signaling messages (i.e., X2AP messages) through the X2 GW. The (H)eNB means an eNodeB or a Home eNodeB. The X2 GW does not terminate X2AP procedures except for the X2AP Message Transfer procedure. That is, X2AP contexts only exist in the two peer (H)eNBs, which is similar to the case where no X2 GW is present. The X2AP contexts define an “X2AP association” between peer (H)eNBs that spans over two SCTP connections.
The X2AP Message Transfer procedure is performed by the X2 GW as follows. That is, when a source (H)eNB sends an X2AP message (except the X2AP MESSAGE TRANSFER message) to a target (H)eNB through the X2 GW, the source (H)eNB encapsulates the X2AP message in an X2AP MESSAGE TRANSFER message, adds routing information (i.e., RNL Header), then sends the X2AP MESSAGE TRANSFER message to the X2 GW. The routing information (i.e., RNL Header) includes both a target (H)eNB ID and a source (H)eNB ID. The X2 GW routes the X2AP MESSAGE TRANSFER message based on the target (H)eNB ID.
The X2 interface is an inter-base-station interface in the 3GPP Release 8 and subsequent releases. The X2 interface includes a control plane (signaling) interface (i.e., X2-C interface) and a user plane (data plane) interface (i.e., X2-U interface). The X2-C interface is used, for example, for preparation of a handover between base stations (i.e., X2 handover), control of Dual connectivity (e.g., establishment, modification, and release of a UE context; and management of an X2 user plane tunnel), and various settings and maintenance related to adjacent eNBs. The X2-C interface uses the X2AP protocol and uses the SCTP and the Internet Protocol (IP) to transfer X2AP signaling messages. The X2AP protocol is referred to as a Radio Network Layer (RNL) protocol and the SCTP/IP, which is used to transfer the X2AP protocol, is referred to as a Transport Network Layer (TNL) protocol.
Meanwhile, the X2-U interface is used, for example, to forward user data packets from a source (H)eNB to a target (H)eNB during a handover, and transfer user data packets (i.e., Packet Data Convergence Protocol (PDCP) PDUs) between a Master eNB (MeNB) and a Secondary eNB (SeNB) in Dual connectivity. The X2-U interface uses the GPRS Tunnelling Protocol User Plane (GTP-U) protocol and uses the User Datagram Protocol (UDP) and the IP to transfer GTP Protocol Data Units (GTP-PDUs). The GTP-U protocol is an RNL protocol for the user plane (U-plane) and the UDP/IP, which is used to transfer the GTP-U PDU, is a TNL protocol for the U-plane. The GTP-U and the TNL UDP/IP provide a tunnel mechanism. That is, the GTP-U encapsulates a user data packet (e.g., IP packet) with a GTP-U header, and the encapsulated user data packet (i.e., GTP-PDU) is transferred on the TNL UDP/IP layer. The user data packet is also referred to as a T-PDU. Further, although the user data packet (i.e., T-PDU) encapsulated with the GTP-U header is one of the GTP-PDUs, it is also referred to as a G-PDU to distinguish it from other GTP-PDUs containing a signaling message between GTP nodes. Further, each of the G-PDU and the GTP-U PDU (i.e., signaling message) is also referred to as a GTP-U message.
Although the 3GPP Release 12 specifies a transfer of X2AP signaling messages (X2-C) between two peer (H)eNBs through an X2 GW as described in Non-patent Literature 1, it does not specify a transfer of user data packets (X2-U). For this matter, Patent Literatures 1 and 2 disclose a transfer of user data packets between two peer (H)eNBs through an X2 GW.
Patent Literature 1 discloses that an HeNB-GW, which relays S1-MME signaling messages and user data packets between a core network (i.e., Evolved Packet Core (EPC)) and an (H)eNB, also supports the X2-C interface and X2-U interface. The HeNB-GW disclosed in Patent Literature 1 operates to relay a GTP-PDU encapsulating a user data packet (i.e., G-PDU) between two peer (H)eNBs. However, Patent Literature 1 does not describe details of the G-PDU relay operation performed by the HeNB-GW.
Patent Literature 2 discloses that a G-PDU is transferred between two (H)eNBs through an X2-GW. The X2-GW disclosed in Patent Literature 2 assigns a Tunnel Endpoint Identifier (TEID) to each of the two (H)eNBs during the preparation of a handover, and operates to change the TEID indicated in the GTP-U header of a received GTP-U PDU from one (H)eNB and send the GTP-U PDU, in which the TEID has been changed, to the other (H)eNB.